Overview: Biological control of weeds

Biological control of weeds is a long-term solution that is most effective when part of an integrated weed management approach.

Plants that have become weeds in Australia are rarely invasive and troublesome in their home country. This is often because populations in the home country are regulated by a variety of natural enemies such as insects and pathogens (disease-causing organisms like fungi and bacteria) that attack the seeds, leaves, stems and roots of the plant.

If plants are introduced to a new country without these natural enemies, their populations may grow unchecked to the point where they become so widespread that they are regarded as weeds.

Crofton weed has been causing grief in Australia since the early 1900s but now the release of a new biological control agent brings hope in managing this invasive pest. Rust fungus on a Crofton weed leaf.

Generally, a weed becomes a problem in the introduced range because its population is above a threshold at which the weed begins to affect the economic or ecological sustainability of the ecosystem.

This, in turn, leads to a decline in the numbers of biocontrol agents until equilibrium is reached between the amount of damage caused by an agent and regeneration by the weed. In a successful biocontrol program this new equilibrium is below the damage threshold that the ecosystem can tolerate.

CSIRO has been working on the biocontrol of weeds since the 1920s, starting with the biocontrol of prickly pear.

It is critical that the biocontrol agents do not become pests themselves. Considerable host-specificity testing is undertaken prior to the release of biocontrol agents to ensure they will not pose a threat to non-target species such as native and agricultural plants.

The initial costs of biocontrol programs are generally high. That’s because we have to find suitable candidate agents overseas, test them for safety in quarantine, and comply with regulations around release.

But once biocontrol agents are released and affect the weeds across its range, follow up control costs are greatly reduced.

CSIRO has many active biocontrol projects underway for both temperate and tropical Australian weeds which cause problems in natural, pastoral and agricultural ecosystems.